1. Field of the Invention
The present invention relates to a technique for acquiring the waveform information of binary data signals of 0 and 1 treated by various kinds of digital apparatuses or transmission systems and effectively evaluating the quality of the eye pattern of the data signals.
2. Description of Related Art
In order to evaluate the digital apparatus or the transmission system, the following methods have been generally used: a method of measuring the bit error rate of the data signal output from an object to be evaluated; and a method of intuitively grasping the degree of fluctuation (jitter) in the phase or amplitude of the data signal while observing the waveform.
As the method of intuitively observing the fluctuation in the phase or amplitude of the data signal with the waveform, a method of displaying an eye pattern has been used.
The eye pattern is obtained by displaying the bit waveforms of the data signal on the same time axis so as to overlap each other. For example, for a data signal x(t) of an NRZ format shown in (a) of FIG. 11, waveform data (H1, H2, . . . ) corresponding to a predetermined number of bits (2 bits) are acquired in synchronization with a trigger signal ((b) of FIG. 11) with a period that is N times the reference bit period Tc of the data signal x(t) and the acquired waveform data overlap each other on the same time axis. In this way, an eye pattern Pe shown in (c) of FIG. 11 is obtained. The actual eye pattern is obtained by displaying the positions corresponding to the size of each waveform data item with points. In FIG. 11, the region in which the points of the waveform data are more likely to be present is hatched and all of the points forming the eye pattern are not included in the hatched portion (which is the same as that in the following description).
The length La of a region A of the obtained eye pattern Pe in the time axis direction depends on the degree of the fluctuation in the phase of the data signal x(t) (the thickness of a level transition portion of data) and the length Lb of the region A in the voltage axis direction depends on the degree of the fluctuation in the amplitude of the data signal (the thickness of a line extending in the time axis direction).
Therefore, the relationship between the eye pattern of the data signal x(t) and the expansion of the region A surrounded by the eye pattern is quantitatively measured to evaluate the quality of the data signal.
As one of the evaluation methods, there is a test method that prepares a mask M that has a polygonal shape (here, a hexagonal shape) corresponding to the shape of the region surrounded by the eye pattern Pe and has a size determined by the bit rate or design amplitude of the data signal and a margin, fixes the mask M to a predetermined position in the vicinity of the center of the region surrounded by the eye pattern, and counts the number of points of the waveform data forming the eye pattern in the mask, as shown in FIG. 12 (mask compliance measurement).
It is defined that, among a total of P points of the waveform data forming the eye pattern, up to Q points are allowed to be present in the mask. When the number of actual points determined to be in the mask is equal to or less than Q, the test result OK (success) is output. When the number of actual points determined to be in the mask is more than Q, the test result NG (failure) is output.
As another evaluation method, there is a test method that prepares a mask M that has a polygonal shape (here, a hexagonal shape) corresponding to the shape of the region surrounded by the eye pattern Pe and has a size determined by the bit rate or design amplitude of the data signal and a margin is used, fixes the mask M to a predetermined position in the vicinity of the center of the region surrounded by the eye pattern, counts the number of points of the waveform data forming the eye pattern while sequentially enlarging the mask M from a standard size, and tests the margin of the quality of the data signal for the mask with the standard size from the relationship between the count value and the enlargement ratio of the mask (margin measurement).
It is defined that, among a total of P points of the waveform data forming the eye pattern, up to Q points of the waveform data are allowed to be present in the mask. From the test result, the enlargement ratio at which the number of points in the mask is more than Q is determined.
An example of the technique for evaluating the quality of the data signal on the basis of the relationship between the eye pattern and the mask is disclosed in the following JP-A-2010-061207.